This invention relates to a method of controlling a tandem rolling mill including a plurality of mill stands which are arranged in tandem for rolling a metal strip. The method minimizes the variation in the tension of the strip when the pass schedule is changed during operation of the mill thus decreasing production difficulties and the quantity of off gauge strip produced at the time the pass schedule is changed.
For the purpose of improving productivity, a rolling mill is generally used to produce strips having different gauges from the same coil of strip or to continuously produce a strip by sequentially interconnecting strips of different coils having the same or different size. To this end, it is necessary to change the pass schedule without stopping the mill and a number of methods of controlling the mill have been developed.
According to one prior art method, when a size changing point reaches a certain stand the set amount of the roll opening of that stand is changed to a value corresponding to the pass schedule for the changed size while at the same time the speed of all of the stands on the inlet side or the exit side of that stand is changed while maintaining the speed ratio at a constant value.
For example, in a tandem mill including (i-1)th, ith and (i+1)th stands as shown in FIG. 1, the size changing point X is shown at a point X immediately on the outlet side of stand i. In this case, let us denote the schedule before size changing by A schedule and that after size changing by B schedule. Then according to this method, since the size changing point X has already reached the ith stand, the roll opening of the (i-1)th stand has already been changed to the set value of the B schedule. On the other hand, the roll opening of the (i+1)th stand is the set value of the A schedule. At this time, the set value of the roll opening of the ith stand should be rapidly changed from the A schedule to the B schedule, the speed ratio between the (i-1)th and ith stands should be changed to that for the B schedule and the speed ratio between the ith stand and the (i+1)th stand should be maintained at the ratio for the A schedule. However, since the strip gauge on the exit side of the ith stand is not equal to that of the A schedule a tension variation caused by the difference in the volume speeds of the A and B schedules reflects on the tension of the strip between the ith and (i+1)th stands. For this reason, the inlet strip gauge and the back tension of the ith stand are the values of the B schedule and different from the target value of the front side tension so that the exit strip gauge is different from the target gauge of the B schedule. In the same manner, with regard to the (i+1)th stand, since the back tension is different from that of the A schedule the exit strip gauge of the (i+1)th stand becomes different from the target value of the A schedule. Thus, the front side tension of a stand which the size changing point has reached must necessarily be changed, and depending upon the extent of the size change the tension variation becomes excessive, thus producing a large quantity of off gauge product.